What is an Acoustic Neuroma?

The acoustic neuroma, also known as vestibular schwannoma,or an acoustic neuroma, is a nonmalignant tumor of the 8th cranial nerve (see Figure 1). Most commonly, it arises from the covering cells (Schwann cells) of the inferior vestibular nerve (Roosli et al 2012). Acoustic neuromas comprise about 6% of all intracranial tumors, about 30% of brainstem tumors, and about 85% of tumors in the region of the cerebellopontine angle. Another 10% are meningiomas. Only about 10 tumors are newly diagnosed each year per million persons in the United States, corresponding to between 2,000 and 3,000 new cases each year. In patients with hearing asymmetry, it is believed that only about 1 in 1,000 has acoustic neuroma (source: National Institutes of Health). Acoustic neuromas are sometimes identified in asymptomatic patients on radiological exams for other reasons, and may be identified in up to 0.02% incidentally (Lin et al, 2005).

What are the Symptoms of Acoustic Neuroma?

Hearing loss is the most frequent symptom, occurring in more than 95% of patients. About 90% present with a one-sided, slowly progressive hearing impairment (see Figure 2). A high-frequency sensorineural pattern is the most common type, occurring in approximately two-thirds of patients. In the remaining third, the next most common observation is hearing loss at low frequency (which would be more typical of Meniere’s disease). Even less commonly, some have the “cookie bite” pattern (suggestive of congenital hearing loss). A sudden hearing loss occurs in about 25% of patients with acoustic neuroma. However, because acoustic neuroma is a rare condition, sudden hearing loss attributable to an acoustic tumor occurs in only 1 to 5% of patients with sudden hearing loss as there are many more common causes (Nosrati-Zarenoe et al 2010, Suzuki et al 2010). Hearing can be completely normal in about 11% of patients (Day et al 2008). Tinnitus is very common in acoustic neuroma, and is usually unilateral and confined to the affected ear.

In spite of the usual origin of acoustics in the vestibular nerve (Roosli et al 2012), vertigo (spinning) is not common, occurring in only about 20% of persons with acoustic neuroma. Vertigo is more common with smaller tumors than larger ones. Unsteadiness is much more prevalent than vertigo, andÂ approximately 70% of patients with large tumors have this symptom. Cerebellar symptoms (that is, poor coordination of the arms and legs) are unusual.

Facial sensory disturbances occur only in large tumors (about 50% of those greater than 2 cm in size). The facial sensory disturbance may respond to carbamazepine medication for neuralgia. Facial weakness is uncommon. Facial twitching, also known as facial synkinesis or hemifacial spasm, occurs in about 10% of patients. Headache prior to surgery occurs in roughly 40% of those with large tumors.

What Causes Acoustic Neuroma?

Acoustic neuroma occurs in two forms: a sporadic form and a form associated with an inherited syndrome. About 95% of all cases are sporadic. The cause of the sporadic form is unclear. Some small studies have found an association of acoustic neuromas with cellular phone use or prolonged exposure to loud noises, but other studies do not find this link (Christensen et al 2004, Edwards et al 2006, Edwards et al 2007, Hardell et al 2003, Lonn et al 2004, Myung et al 2009, Schlehofer et al 2007, Schoemaker et al 2007). There is not hard evidence supporting a link between environmental factors and acoustic neuromas.

The inherited syndrome called neurofibromatosis type II (NF2). NF2 is rare; there are only several thousand affected individuals in the entire United States, corresponding to about 1 in 40,000 individuals. Roughly 5% of patients with acoustic neuroma have NF2.

How is Acoustic Neuroma Diagnosed?

Figure 2: MRI scan of the brain showing an acoustic neuroma (the white spot on the left side of the picture).

Conventional audiometry is the most useful diagnostic test for acoustic neuroma. The most common abnormality is an asymmetrical high-frequency sensorineural hearing loss. However, recall that about 1 in 1,000 patients with hearing asymmetry have acoustics. It has been estimated that 5% of persons with sensorineural hearing loss have acoustics (Daniels et al 2000), but this estimate is suspect as it would imply a much higher prevalence of acoustic neuromas than are commonly accepted. Less conservative incidence estimates for acoustic neuroma in asymmetrical hearing loss are reported at 2.5% (Suzuki et al 2010). When abnormal with a progressively worsening pattern, audiometry usually leads to further testing such as ABR (auditory brainstem response) and gadolinium enhanced MRI (magnetic resonance imaging), which establishes the diagnosis. ABR testing is less sensitive than MRI, but it is considerably less expensive. A new technique, called summated ABR, which is essentially several ABRs compared over time, may provide better sensitivity.

Electronystagmography (ENG testing) is frequently abnormal, and about 50% of all tumors are associated with unilateral loss of calorics. Nevertheless, ENG is not a reasonable diagnostic test because it is not specific. Rotatory chair testing is less sensitive than caloric testing. Posturography is insensitive to acoustic neuroma.Figure 3: Typical audiogram (hearing test) in a patient with an acoustic neuroma.

Although it is relatively costly compared to audiometry or ABR, the optimal test for excluding an acoustic neuroma is a gadolinium enhanced T1 MRI (Figure 3). On MRI, acoustic neuromas are frequently uniformly enhanced, dense, and expand the internal auditory meatus. A fast spin-echo T2 variant of MRI is very sensitive to acoustics, and in some clinical settings, can be done fairly inexpensively. If an MRI cannot be done, an air-CT scan should be obtained in high-risk individuals, particularly if the ABR is suggestive of an acoustic neuroma.

Acoustic neuromas range in size up to 4 cm.Â The smallest, the intracanalicular acoustic, is measured in millimeters. A “small” acoustic is less than 1.5 cm, a “moderate” acoustic is 1.5 to 3 cm, and a “large” acoustic is 3 cm or greater. Tumors are staged by a combination of their location and size: An intracanalicular tumor is small and in the internal auditory canal (IAC). A cisternal tumor has extended outside the IAC. A compressive tumor is touching the cerebellum or brainstem. Some tumors cause hydrocephalus by obstructing cerebrospinal fluid (CSF) drainage pathways in the 4th ventricle.

Rarely, acoustic neuromas are inherited. Acoustic neuroma caused by neurofibromatosis type II (NF 2) should be suspected in young patients and those with a family history of neural tumors. There are several other tumors that can occur in the same region of the brain [the cerebellopontine angle (CPA)] as acoustic neuromas. Of all lesions in the CPA, acoustic neuromas account for 70 to 90%. Meningiomas are second most common (10%), followed by epidermoids, and then lipomas.

Gamma Knife

This is a method of irradiating the tumor, invented by Lars Leksell in 1971. Gamma knife stereotactic radiosurgery has become more prevalent recently as it has been demonstrated to be safe and effective in the control of acoustic neuromas (Likhterov, 2007). Gamma knife does not generally make tumors go away — Figure 3 is actually that of a patient who had gamma knife surgery several years prior. Instead, gamma knife radiation shrinks the tumor and prevents future growth in most patients. Patients are best followed with periodic MRI scans for the remainder of their lives. The recurrence rate of the tumor is about 3% after surgery, and 14% after gamma knife, but of course, this figure will vary with the surgeon and the gamma knife protocol.

If surgery is eventually required, surgical complications in this situation, such as severe facial nerve weakness, are nearly 100%. This occurs because the facial nerve often becomes “fused” to the tumor after the gamma knife procedure. Like surgery, hearing loss is common after gamma knife surgery. The risk of hearing loss correlates with the radiation dose to the cochlea (Thomas, 2007; Massager, 2007). Delayed facial weakness, and facial numbness also occur in roughly one-third of patients after gamma knife. Hydrocephalus has been reported to occur in between 3 and 12.8% (Noren et al, Pollock et al). Dysequilibrium is reported 8 to 31% of the time, a figure analogous to surgical management. The long-term complication rate of gamma knife treatment has not been well documented. Radiation to the head increases the risk of future tumors of the brain and skull, but this risk is thought to be small. Radiation damage can also occur in the surrounding tissues, including nerves that control the face and tongue. A five year follow-up study of 317 patients who underwent gamma knife radiosurgery found a five- and ten-year progression-free survival rates of 93% and 92% with minimal complications.

Cochlear Implantation

Very rarely, a person with acoustic neuroma might desire a cochlear implant. This might occur if an acoustic tumor is present in the only hearing ear, or after surgery to remove bilateral acoustic neuromas. Belal (2001) reported that cochlear implantation is possible only if there is an intact cochlear nerve (as shown by a positive response to promontory stimulation), and if the implantation is done at the time of acoustic tumor removal, before the cochlea ossifies (turns to bone).

How Might Acoustic Neuroma Affect My Life?

Persons with acoustic neuroma may experience hearing loss, imbalance, and facial weakness. Those with extremely large tumors may also have other neurological problems. Many persons with acoustics will eventually need brain surgery to remove the tumor. Those who opt not to have surgery will likely need to have periodic imaging studies to determine if it is still safe to leave the tumor without treatment.

Research Studies in Acoustic Neuroma

In January 2008, a visit to the National Library of Medicine’s search engine, PubMed, revealed more than 5409 research articles referring to acoustic neuroma published since 1949 with 150 published in the last year. In spite of this concentration of effort by the medical community, acoustic neuroma remains a disorder that cannot be prevented and is often diagnosed after it is too late to save hearing. At the American Hearing Research Foundation (AHRF), we have funded basic research on acoustic neuroma in the past, and are very interested in funding additional research on acoustic neuroma in the future. We are particularly interested in projects that might lead to early detection (prior to onset of hearing loss). Donate to AHRF’s efforts to detect and treat acoustic neuroma.

Acknowledgments

Graphics in Figure 1 and Figure 2 are courtesy of Northwestern University (Chicago IL).

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The American Hearing Research Foundation is a non-profit foundation that funds research into hearing loss and balance disorders related to the inner ear and is also committed to educating the public about these health issues.